You will certrainly get cover from Farnborough just to the west of Lydd at any reasonable height. I have many times switched to Farnborough on the climb out west bound through 3,000 feet and worked Farnborough fine.
Going eastbound towards Headcorn Farnborough will provide you with cover to the otherside of Headcorn but not quite as far as Canterbury (level dependant) and will often tell you, you are approaching the edge of their cover, but they will carry on providing a service if you ask. Manston will cover you from Canterbury down to east of Lydd when their head is operational. If things are murky and you need it Gatwick will work you well east of their zone.
All in all their is a corridor between Manston, Gatwick and Farnborough where you may not get cover but it is quite small. Of course if Manston is in op once you get east of Canterbury you might just as well be in the wilds of Scotland, and God forbid you are forced to land - the natives will almost certainly eat you.